Concrete generally refers to a mixture of natural and/or artificial aggregates, such as, for example, sand and either a gravel or a crushed stone, which is held together by a binder of cementitious paste to form a highly durable building material. The paste is typically made up of a hydraulic cement, such as Portland cement, and water and may also contain one or more chemical admixtures as well as supplementary cementing materials, such as, for example, fly ash or ground granulated blast furnace slag cement.
Early cements were based on calcined lime, which is produced by exposing limestone at an elevated temperature, for example, a temperature well in excess of 800° C., in the presence of an oxygen-containing atmosphere to form quick-lime according the reaction in equation (1).CaCO3→CaO+CO2(g)  (1)
Hydraulic limes are derived from calcined limes that have some amount of clay. The clay provides silicon and aluminum that react with the calcium from the limestone to produce cements having complex compounds that hydrate. These compositions even have the ability to harden underwater. Portland cement eventually evolved from these materials.
Most construction cements today are hydraulic, and most of these are based on Portland cement. Hydraulic cements set and harden after being combined with water, as a result of chemical reactions induced by the water, and demonstrate an improved strength and stability even under water after hardening.
Setting and hardening of hydraulic cements is caused by hydration reactions that occur between the compounds that make up the cement and water, which result in the formation of hydrates or hydrate phases. The cementitious composition begins to progressively stiffen leading to the onset of setting, where additional consolidation of the hydration reactants occurs. Hardening follows setting, which is characterized by a steady growth in the compressive strength of the material over a period that can range from a few days in the case of “ultra-rapid-hardening” cements to several years in the case of ordinary cements.
Portland cement consists of five major compounds as well as some additional minor compounds. The major compounds are tricalcium silicate, 3CaO.SiO2; dicalcium silicate, 2CaO.SiO2; tricalcium aluminate, 3CaO.Al2O3; tetracalcium aluminoferrite, 4CaO.Al2O3.Fe2O3; and gypsum, CaSO4.2H2O. The hydration of tricalcium silicate is represented by the reaction according to equation (2).2(3CaO.SiO2)+11H2O→3CaO.2SiO2.8H2O+3Ca(OH)2  (2)Upon the addition of water, the reaction rapidly progresses to release calcium and hydroxide ions. Once the water solution becomes saturated, the calcium hydroxide begins to precipitate forming a crystalline structure. Calcium silicate hydrate is also simultaneously formed. As the calcium hydroxide precipitates from solution, the tricalcium silicate continues to react to form calcium and hydroxide ions. The reaction is somewhat exothermic involving the evolution of heat as the reaction progresses.
The formation of calcium hydroxide and calcium silicate hydrate provides “seeds” around which calcium silicate hydrate may continue to form. At a certain point, the rate of reaction finally becomes controlled by the rate of diffusion of water molecules through the layer of calcium silicate hydrate that surrounds the unreacted tricalcium silicate, which progressively becomes slower as the layer of calcium silicate hydrate grows larger.
Dicalcium silicate is hydrated to form the same products as tricalcium silicate according to the reaction in equation (3).2(2CaO.SiO2)+9H2O→3CaO.2SiO2.8H2O+Ca(OH)2  (3)However, the hydration of dicalcium silicate occurs much more slowly and is mildly exothermic in comparison to that for tricalcium silicate.
The reactions of the other major components of Portland cement are more complex and beyond the scope of the background discussion given here. However, the hydration of cement experiences five distinct phases. Phase I is characterized by rapid hydrolysis of the cement compounds and can result in a temperature increase of several degrees over a period of on the order of 15 minutes. The evolution of heat begins to dramatically slow in phase II, the dormancy period, which can last from one to three hours. In phases III and IV, the concrete begins to harden and the evolution of heat begins to increase due primarily to the continued hydration of tricalcium silicate. These phases can encompass a period of up to approximately 32 to 36 hours. Stage V marks a period of continued hydration, but at much lower rates, and continues as long as unreacted water and unhydrated silicates remain and can come in contact with one another. Stage V typically continues on the order of days, if not longer.
More commonly, modern-day cements are formulations of hydraulic cement blends. For example, a hydraulic cement, such as, for example, Portland cement, can comprise up to 75% of ground granulated blast furnace slag that results in a reduction in early strength but provides increased sulfate resistance and diminished heat evolution during the stiffening and hardening stages of the concrete.
Blended hydraulic cements can comprise one or more pozzolan materials, which are siliceous or aluminosiliceous materials that demonstrate cementitious properties in the presence of calcium hydroxide. The silicates and even aluminates of a pozzolan reacting with the calcium hydroxide of a cement form secondary cementitious phases (e.g., calcium silicate hydrates having a lower calcium to silicon ratio), which demonstrate gradual strengthening properties that usually begin to be realized after 7 days of curing.
A blended hydraulic cement may comprise up to 40% or more fly ash, which reduces the amount of water that must be blended with the cementitious composition allowing for an improvement in early strength as the concrete cures. Other examples of pozzolans that can be used in hydraulic cement blends include a highly reactive pozzolan, such as, for example silica fume and metakaolin, which further increases the rate at which the concrete gains strength resulting in a higher strength concrete. Current practice permits up to 40 percent or higher reduction in the amount of hydraulic cement used in the concrete mix when replaced with a combination of pozzolans that do not significantly reduce the final compressive strength or other performance characteristics of the resulting concrete.
The cementitious materials in concrete require water, typically known as chemical water or hydration water, to chemically evolve into a hard, crystalline binder. For example, Portland cements generally require up to about 40% of their weight as water in order to promote complete hydration and chemical reaction.
Excess water has conventionally been added to make concrete more plastic allowing it to flow into place. This excess water is known as water of convenience. A small amount of the water does escape as a result of solids settling during the plastic phase, evaporation at the atmospheric interface, and absorption into accepting interface materials. However, most of the water of convenience remains in the concrete during and immediately following hardening. The water of convenience can then escape into the atmosphere following the hardening of the concrete. The water of convenience may represent up to about 70% of the total water in the concrete.
The concrete construction and floor-covering industries may incur both construction delays and remedial costs as a result of water vapor emissions and water intrusion from concrete. For example, adhesives and coatings used in the construction of concrete floors are relatively incompatible with moisture that develops at the concrete surface. Moisture may also create an environment for promoting the growth of mold.
Water tightness in concrete structures is a measure of the ability of the hardened concrete to resist the passage of water. Water vapor emission is proportional to the state of relative dryness of the body of the concrete structure. Once isolated from external sources of water, water vapor emissions are derived from the amount of water that is used in excess of that needed to harden the cementitious materials, i.e., the water of convenience. Depending upon the atmospheric temperature and humidity at the surface and the thickness of the concrete, the elimination of excess water through water vapor emissions can take on the order of many months to reach a level that is compatible with the application of a coating or an adhesive.
Installation of an impermeable barrier on the surface of the concrete prior to reaching an acceptable level of dryness may result in moisture accumulation, adhesive failure, and a consequential failure of the barrier due to delamination. Premature application of coatings and adhesives increases the risk of failure, while the delay caused by waiting for the concrete to reach an acceptable level of dryness often results in unacceptable construction delays.
The floor covering industry has determined, depending on the type of adhesive or coating used, that a maximum water vapor emission rate of from 3 to 5 pounds of water vapor per 1,000 square feet per 24 hour period is representative of a state of slab dryness necessary before adhesive may be applied to the concrete floor.
There remains a need in the art for cementitious compositions that reduce the amount of time needed to reach a desired water vapor emission rate in concrete floors enabling a more timely application of coatings and adhesives.
It is known in the art that certain polymers classified as superplasticizers may be included in cement in order to reduce the amount of water of convenience needed to allow the cementitious mix to more readily flow into place. Certainly, a reduction in the amount of excess water remaining after the concrete hardens should lead to a reduction in the amount of time necessary to reach a desired water vapor emissions rate. However, the use of superplasticizers alone does not address other effects that influence the rate of water vapor emission from the hardened concrete.
There remains a need in the art for cementitious compositions that further reduce the amount of time necessary to reach a desired water vapor emission rate in concrete floors beyond that which is achieved through reduction in the amount of water required through the use of a superplasticizer additive.